Use a mirrored mirror log

Use a mirrored mirror log

By default, LVM will sync the two mirrors of a logical volume on
creation. This is I/O intensive and it takes a long time though, so we
tell it not to with --nosync. The next default behavior is to create a
mirror log which says what portions of the mirrors are in sync. But
then if a disk backing a mirror log fails, we have to deal with
resyncing the mirrors at each reboot.

This commit tells LVM to use a mirrored mirror log, which will use
approximately 4MB of extra space, but means we'll never have to
sync mirrors, ever.

Change-Id: I6586ced4e7e66ae79ae134406a4c633f8fbc8602